1. Field of the Invention
The present invention relates to a motorized retractor which is employed at a seat belt apparatus for restraining the body of an occupant sitting on a seat of a vehicle or the like with a webbing, and which enables winding and unwinding of the webbing.
2. Description of the Related Art
A seatbelt apparatus which restrains an occupant sitting in a seat with a long belt-form webbing is attached to a vehicle such as a passenger car or the like. Among such seatbelt apparatuses, a “three-point”-type seat belt apparatus is equipped with a retractor (a webbing winding device) which stores the webbing in a wound state and retains the webbing to be unwindable.
The retractor is provided with a winding spool and an urging member, such as a spiral spring or the like. A length (longitudinal) direction base end side of the webbing is anchored at the spool, and the webbing is wound onto the spool, from the base end side, by rotation of the spool. The urging member urges the spool in a direction for winding up the webbing. At a seat belt apparatus which is equipped with this retractor, when an occupant has applied the webbing, the spool is urged in the direction for winding up the webbing by urging force of the urging member. As a result, slack of the webbing is taken up and the webbing restrains the occupant. Hence, when the occupant releases the state of application of the webbing, the webbing is wound onto the spool by the urging force of the urging member.
At this time, if the urging force of the urging member is too small, the webbing ends up in a slack state rather than being completely wound up, which causes a deterioration in appearance at times of non-use of the webbing. On the other hand, if the urging force of the urging member is too large, this can cause an occupant to whom the webbing is applied to experience a sense of constriction.
Accordingly, motorized retractors (motor-type seatbelt retractors) have been considered (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-163186). In a motorized retractor, urging force of the urging member is reduced, to relieve or suppress the sense of constriction experienced by an occupant, and the motorized retractor is equipped with a mechanism (a “winding-assistance mechanism”) which drives the spool with driving force of a motor, in order to compensate for a lowering in the force for winding the webbing onto the spool that results from the reduction of the urging force.
A motorized retractor (seatbelt apparatus) equipped with such a winding-assistance mechanism has a structure in which, when a buckle switch senses that engagement of a buckle with a tongue plate has been released, that is, when an occupant releases a state of application of the webbing, the motor operates for a certain amount of time and turns the spool in the direction for winding up the webbing.
However, in a case in which urging force of the urging member is reduced to relieve the sense of constriction experienced by an occupant, as described above, this reduction causes a slight slackness of the webbing in the applied state, and results in a reduction of restraining effectiveness of the webbing at a time of vehicle collision or the like.
Therefore, a motorized retractor has been considered which is equipped with a mechanism (a “pretensioner mechanism”) which improves restraining effectiveness of the webbing by forcibly rotating the spool in the winding direction with driving force of the motor when a risk of collision is detected, (see, for example, JP-A No. 2001-347923).
A motorized retractor that is equipped with such a pretensioner mechanism has a structure which detects a state of rapid deceleration of the vehicle with an acceleration sensor, and forcibly rotates the spool in the winding direction with the driving force of the motor in accordance with an electronic signal from the acceleration sensor.
Now, there have been calls for development of a motorized retractor that is equipped with a combination of the two mechanisms described above, the winding-assistance mechanism and the pretensioner mechanism. For such a case, because, as described above, the two mechanisms both have structures which wind the spool in the winding direction with driving force of a motor, it is preferable to provide a single motor for the two mechanisms.
However, there is a problem in that capabilities that are required of the motor, a reduction mechanism of the motor and so forth are mutually contradictory between the winding-assistance mechanism and the pretensioner mechanism.
Specifically, in a motorized retractor which is equipped with a winding-assistance mechanism, in consideration of safety of an occupant and the like, a winding torque on the spool that results from driving force of the motor is preferably set to a low level for supplementing the urging force of the urging member. Therefore, the reduction mechanism is specified with a low reduction ratio.
In contrast, in a motorized retractor which is equipped with a pretensioner mechanism, it is necessary to forcibly wind up the webbing in opposition to an inertial force of the occupant, who will be acting to move toward a front side of the vehicle because of the rapid deceleration of the vehicle. Therefore, it is necessary to set the winding torque on the spool from the driving force of the motor to be high. Accordingly, the reduction mechanism is specified with a high reduction ratio.
Consequently, it has been difficult to provide the two mechanisms, the winding-assistance mechanism and the pretensioner mechanism, with a single motor (and reduction mechanism). In the meantime, in cases in which dedicated motors and reduction mechanisms have been separately provided for the two mechanisms, there have been problems in that costs are higher and the bodies of the motorized retractors are larger.